1. Field of the Invention
The present invention relates to interferometry, correlation and autocorrelation of wide bandwidth signals, and more specifically the coherent conversion of wide bandwidth signals to a set of many lower frequency signals and subsequent parallel signal processing.
2. Description of Related Art
Electromagnetic waves such as microwaves and light, and ultrasound waves are widely used for echolocation, velocimetry and imaging. These techniques involve signal processing such as correlation, interferometry, time delaying, filtering, recording and waveform synthesis. Currently, many of these applications use fairly monochromatic waves having a long coherence length. If instead wide bandwidth waves are used, an increased precision of the location and velocity of the target results due to the illumination's shorter coherence length. U.S. Pat. No. 5,642,194, titled "White Light Velocity Interferometer" and U.S. patent application Ser. No. 08/720,343, titled "Noise Pair Velocity and Range Echo-Location System", both incorporated herein by reference, describe how two interferometers nearly matched in delay can be used with wide bandwidth illumination to measure target velocity using processes of interferometry and autocorrelation (FIG. 2A). The wideband illumination can be used to measure target range using the process of correlation (FIG. 2B). These processes of interferometry, correlation and autocorrelation require coherent delays. For microwave radars, the delays are chosen to be several milliseconds to match radar pulse repetition rates, which in turn are set by the desired maximum target range of hundreds of kilometers. Several milliseconds is a long delay compared to the period of the wave, which could be 30 picoseconds. Electronics, and particularly digital electronics, is the most attractive method of creating long delays, as well as for performing general signal processing. The challenge is that the bandwidths found in microwave and optical signals can greatly exceed the capability of easily available electronics.
For example, suppose we wish to construct a device using 10-30 GHz microwaves having a bandwidth of 20 GHz, using interferometers of 2 ms delay. (This could provide meter/sec velocity measurement and .about.1 cm range resolution.) It is impractical to create such a long delay by analog cable, because the cable length required is hundreds of km, and serious attenuation and dispersion of the signal would result, in addition to the impractical weight and cost of the cable. A digital delay line consisting of an analog-digital converter (A/D), shift register, and digital-analog converter (D/A) can easily create a 2 millisecond delay. However, the 20 GHz input signal bandwidth greatly exceeds the bandwidth capability of commonly available digital electronics, which may be closer to 0.2 GHz.
The bandwidth of optical signals can be even much higher than microwaves and thus even further beyond the capability of current electronics. (Optical velocimeters having long delays are useful for measuring extremely slow velocities.) In spite of lower weight of optical fiber, a spool of fiber hundreds of km long is still large and expensive, and the attenuation and dispersion through such a length would force the use of repeaters, further increasing cost and weight.
In addition to velocimetry, applications for correlating wide bandwidth optical signals abound in astronomy, for creating long baseline optical telescopes analogous to radio telescope arrays. These could increase the angular resolution by orders of magnitude over current optical telescopes. The challenge here is to correlate two or more optical signals received many hundreds of kilometers apart. Propagating a weak optical signal through long optical fibers brings severe attenuation and dispersion, losing the phase information required for correlations. Digitally encoding the time varying optical field (amplitude and phase) was not feasible previously.
A high bandwidth requirement can also come from the use of multiple input data streams which need to be correlated to form a 2-dimensional image. Such is the case of an ultrasound imaging device using multiple detectors (transducers) and using wideband illumination (to reduce speckle). While a single detector produces a 1 to 10 MHz bandwidth signal, which is low enough to be processed by a single channel of electronics, a matrix of 100 such detectors requires calculating a large number of correlation combinations. This can exceed real-time processing capacity.
FIGS. 2A, 2B, and 2C show applications for interferometry, autocorrelation and correlation. FIG. 2A shows a double interferometer velocimeter, which is also called a "white light" velocimeter for optical waves (see U.S. Pat. No. 5,642,194) and a "noise pair" radar for microwave waves (See U.S. patent application Ser. No. 08/720,343). Waves from a wideband source 100 pass through an interferometer 102 labeled "A" having a delay .tau..sub.A. This imprints a coherent echo on the source to form an illumination beam 104, which is sent to a target 115. The presence of the coherent echo causes a comb-filter shape to the power spectrum of waves 104. The waves 106 reflected from the target are Doppler shifted by the target velocity, which dilates the comb-filter spectrum slightly. The reflected waves pass through a second interferometer 108 labeled "B" having a delay .tau..sub.B and which imposes its own comb-filter pass spectrum. This creates a detected wave 110. The time-averaged power 112 of the detected wave is measured versus delay .tau..sub.B to form an autocorrelation. The autocorrelation will have a peak for delays .tau..sub.B .apprxeq..tau..sub.A. This is because the maximum amount of power will pass through the "B" interferometer when its spectrum matches the Doppler shifted spectrum of the "A" interferometer. Let the position of the peak be .tau..sub.p. For target velocity v, and for a target reflecting light 180.degree. back toward the apparatus, the autocorrelation peak shift is proportional to velocity, .tau..sub.p -.tau..sub.A =(2v/c).tau..sub.A, where c is the wave speed. Thus very long delay times produce sensitive velocity detection. The advantage of high bandwidth for the source 100 is a smaller coherence length, which makes the autocorrelation peak narrower, reducing velocity ambiguity. This allows resolving velocities of multiple targets.
An interferometer is a device that coherently sums an applied signal with a delayed replica. It is a filter with a power transmission spectrum which is sinusoidal, as shown in FIG. 3A, ideally given by T=(1/2)1+cos(.omega..tau.+const)!, where "const" is a constant, .omega.=2.pi.f is the angular frequency, and .tau. the interferometer delay. This kind of spectrum is also called a "comb-filter". FIG. 3B shows a close-up of the sinusoidal peaks having a periodicity 1/.tau. in frequency units and 360.degree. in phase units. (The periodicity of the sinusoids in these Figures is exaggerated for clarity. For example, a 2 ms delay interferometer would have 2 ms.times.20 GHz=40 million peaks from 10 GHz to 30 GHz.)
Note that it is the structure of the comb-filter on the scale 1/.tau. which produces the velocity measuring effect. Spectral changes much broader and much narrower than 1/.tau. do not significantly affect the phase of the fringes. Thus a less-than-ideal interferometer that broadly modifies the spectrum of the applied signal can still be used, as well as one having slightly non-uniform spacing (phase) of comb peaks, provided both "A" and "B" interferometers share the same non-uniformity.
FIG. 2B shows an application of high bandwidth signals to find the range of a target, and FIG. 2C to measure the angle of a target by the difference in arrival times of a wave at two antenna spaced well apart (such as in radio astronomy). These applications both create a correlation between two signals "A" and "B". The position (in delay-space) of the correlation peak yields the range or angle information. Again, the advantage of using a high bandwidth source is to create a narrow correlation peak, which reduces ambiguity of the measurement.
In addition to correlations and interferometry, other signal processes which benefit from high bandwidth are the recording and synthesis of waveforms. The time resolution of these processes is given by the reciprocal of the bandwidth, so high bandwidth allows for the shortest time resolution.